Alkanoate esters of hydroxybenzenesulfonic acids and salts thereof are useful as bleach activators in detergent compositions (Allan H. Gilbert, Detergent Age, 1967, June, pages 18-20 and August, pages 30-33). These ester compounds, also named as alkanoyloxybenzenesulfonic acids and salts thereof, typically are manufactured by contacting a hydroxybenzene-sulfonic acid or salt thereof with a saturated, aliphatic carboxylic (alkanoic) acid, usually an alkanoic acid containing 6 or more carbon atoms, or an ester-forming derivative thereof such as an anhydride or acid halide, under ester forming conditions. Such processes are carried out either in a solvent or the carboxylic acid related to the desired ester product at temperatures of 80 to 200.degree. C. These known methods are well summarized in U.S. Pat. Nos. 4,587,054, 4,588,532, 4,883,612 and 5,069,828. The preferred route for commercial scale synthesis is esterification of sodium 4-phenolsulfonate (SPS) using carboxylic acid anhydrides as the esterification agent. SPS is manufactured by the sulfonation of phenol which initially produces a relatively large (.apprxeq.40 %) amount of the ortho-hydroxybenzene sulfonic acid. Heating the mixture of ortho- and para-hydroxybenzene sulfonic acids produces a high para (&gt;95%) product, which is neutralized to yield SPS.
An alternative to these methods is the sulfonation of aryl carboxylate esters. An advantage of this alternative is that the sulfonation of a phenyl ester occurs with higher selectivity for the desired para sulfonation product than the sulfonation of phenol described above (see Ansink and Cerfontain Recl. Trav. Chim. Pays-Bas 1992, 111, 215). U.S. Pat. Nos. 4,695,412 and 5,124,475 disclose the sulfonation of pre-purified aryl esters with sulfonating agents such as sulfur trioxide, oleum or chlorosulfonic acid to produce the desired alkanoate esters of hydroxybenzene sulfonic acids. The methods described in these patents first require the synthesis and purification of an aryl alkanoate ester. U.S. Pat. No. 3,772,389 describes the prior art in the synthesis and manufacture of aryl esters and discloses a process in which a carboxylic acid and a phenol are combined between the temperatures of 75-285.degree. C. using a borate-sulfuric acid catalyst. E. J. Bourne and coworkers in the Journal of the Chemical Society 1949, 2976-79 disclose the use of trifluoroacetic anhydride (TFAA) as an "impeller" in the synthesis of aryl alkanoate esters using milder conditions. It has been discovered that the effectiveness of the TFAA impeller method is increased when trifluoroacetic acid (TFA) is used as a solvent, in particular for the synthesis of hydroxybenzenesulfonate esters, and that the reactions carried out in this fashion occur in very high yield (&gt;95%). In addition, because these TFAA impeller esterificatons occur at ambient temperatures in minutes rather than hours the common problems of color body or other byproduct formation are minimized.
Sulfonation reactions carried out in TFA are not well known but U.S. Pat. No. 2,628,253 discloses that TFA reacts with SO.sub.3 to yield the mixed anhydride having the structure ##STR1##
Bert H. Bakker and Hans Cerfontain report in the Eur. J. Org. Chem., 1999, 1, 91-96 that this mixed anhydride reagent can be used for the sulfonation of alkenes. Tyobeka, et al., Tetrahedron, 1988, 44, 1971-78, further discloses that the bis-mixed anhydride of TFA and sulfuric acid: ##STR2##
is produced in equilibria by contacting TFAA with sulfuric acid.